What does crop factor compare?

• A. The ratio between the sensor diagonal and the full-frame diagonal
• B. The ratio of focal length to sensor width
• C. The ratio of shutter speed to ISO
• D. The ratio of image resolution to sensor size

Answer: (A)

Crop factor measures how a camera’s sensor size compares to a full-frame sensor, using a single number based on size. We express this by looking at diagonals, since the diagonal is a compact way to capture overall sensor area. The ratio of the full-frame diagonal to the sensor diagonal (or equivalently the sensor diagonal to the full-frame diagonal) tells you how the field of view changes with the same lens on different bodies. For example, a crop factor of 1.5 means a 50mm lens on that smaller sensor gives roughly the same field of view as a 75mm lens on full-frame. The option matches this idea by describing the ratio between the sensor diagonal and the full-frame diagonal.